The transmission of electrical power from an electric generator to residential areas involves a combination of transmission devices which make up a transmission system. In a typical transmission system, power is generated by a hydroelectric installation, a steam installation or a nuclear plant. The output from the generator is normally approximately 25 kv. The output from the generator is transmitted to a step up substation where the voltage is increased to a transmission line voltage of 230 kv or higher. The next substation encountered is a transmission substation where the transmission voltage is decreased from the transmission line voltage to a subtransmission voltage of approximately 69 kv. A distribution substation is then used to step the voltage down from the transmission voltage to a distribution voltage of 5 to 35 kv. The distribution voltage is the voltage that is transmitted to the residential area either through overhead or underground distribution systems. Single phase transformers are provided at the residential level to reduce the voltage to the 240-120 single phase, three wire residential power entrance.
Because of the increased costs for labor, equipment and land, there is a need for finding ways to reduce the total costs at each of the power distribution levels. One of the levels which can be improved to reduce costs and preserve land is at the distribution substation level. Through habit and convention, it has been the practice of utilities to engineer their own substation designs, purchase the required components from a variety of sources, assemble and test those components at the substation site. One of the several problems with this approach is the equipment delivery schedule. This means that they may get the transformer in 26 weeks, but the switch gear may take 40 or 50 weeks. This increases cost, since the transformer must be carried as an inventory item until the switch gear is ready for assembly. The problem is compounded when the number of suppliers of the components that are to be used in the substation increases. Another problem, the components from individual manufacturers are not interchangeable and often modifications must be made at the site in order to adapt the various components to the system. Also, because the components are purchased from various manufacturers, manufacturing responsibility is divided, often resulting in delays in determining manufacturing responsibility for failure or disruption of service from the substation.
Large land areas are normally required for this type of substation since the various components are individually mounted on separate foundations within the substation complex and proper clearances must be maintained for safety and maintenance procedures.
A key requirement for the electrical industry is to maintain reliability of the electrical system so that the number of hours of outage or down time at the residential level is kept to a minimum. The goal is to provide service at the residential level with a minimum number of hours of down time.
As the demand for electric service, and the cost of fuel has increased, the utilities have considered raising the present voltage level of the distribution voltage system up to the sub-transmission voltage level. The reason for this is that the losses in the transmission line are proportional to the square of the current. As the voltage is increased with the same load expressed as MVA, the current decreases and therefore the losses in the transmission line decrease so the amount of power that can be delivered to the residential area is increased. This results in reduced operating costs.
A normal way to remove a transformer from service in a distribution substation is to use a disconnect switch with power fuses. Because of relay coordination problems, the power fuses are normally set quite high, so that the transformer will be subjected to this abnormality for a relatively long period of time before it can be cleared. Also, the fuses clear on a single phase basis, and do not permit the three phase de-energization of a transformer. With the right circumstances of capacitance, secondary voltage, and a lightly loaded transformer, this can cause a resonance between a magnetizing reactance of the transformer and the capacitance of the distribution system, and this can result in overvoltages on the transformer which would cause arcing and external flashovers. These could be dangerous to anyone in the substation area, and could result in a substation fire.
A substation generally employs a circuit breaker on the low voltage side of the transformer to remove load from the transformer in the event there is a fault. With a breaker on the secondary, fuses are felt to be adequate protection for the primary. However, it is not suitable to supply a differential relay to protect the transformer, as this relay senses internal faults in the transformer, therefore, the only way to effectively remove this internal fault is to remove the transformer from service with a three phase disconnect switch.
The transformer secondary is normally connected to some circuit breakers or other devices that can be used to switch the feeder circuits to the distribution system. This switchgear can be metal clad switchgear or an outdoor substation using oil circuit breakers, or it might possibly be merely a recloser. The outdoor substation is quite bulky, but it does offer maintenance advantages as the breakers can be individually bypassed; but it does require a large area of land. Metal clad switch gear is popular for industrial applications, but it is generally limited to 15 KV applications and has only recently becoming available at 25 and 35 KV. The reclosure is limited to approximately 600 Amps and is used basically in rural systems or on low priority loads.